the oades battery is a nimh battery, the walmart is a lithium ion. The walmart battery is nice, in that it has a built-in charger, etc. However, lithium batteries lose capacity over time, while nimh batteries are less susceptible to this.
If you're comfortable making the mods necessary to have the walmart (or rc car battery) solution work, go for it. I'm sure the oade's battery solution will be a well made (and likely fused) companion to the ua5 and will work out of the box (and will be supported by the oades). A diy solution will not receive such support.